Publication: The Journal of Electricity
San Francisco, CA, United States
INSULATORS FOR USE ON HIGH VOLTAGES.
BY HENRY D. SEARS.
(A paper read before the Santa Cruz Convention of the Pacific Coast Electric Transmission Association, August 17, 1897.)
The apparatus for the development and conversion of electricity at high voltages, having reached a satisfactory degree of perfection, the question to be considered is that of the line. The only problem here is that of insulation. Bare wire is universally used and thus the entire burden is thrown on the insulating supports which carry the conductors and absolute and entire dependence must be placed on them. It is therefore evident that the question of insulators is of the most vital importance.
To properly serve their purpose these insulators should fulfill a number of conditions.
1. They should be made of an absolute non-conductor.
2. They should not absorb moisture.
3. They should be unaffected by exposure to atmospheric influences.
4. They should be of great strength.
5. They should be made of material to the surface of which moisture, dust, smoke, etc., will not adhere.
6. They should he of such shape as to readily shed water.
7. They should be of such size and shape as. to reduce surface leakage to a minimum.
The only materials seriously considered for the manufacture of insulators, to fulfill the above conditions, are porcelain and glass. In two very important points porcelain is so far superior that it has been adopted on every line which is actually transmitting power in quantity. These two points are,
1. Strength and brittleness. It takes a much harder blow to break a piece of porcelain than an equivalent piece of glass, and when the blow is hard enough to be effective, porcelain merely chips while glass completely shatters. Insulators are the special targets for small boys with stones and for larger ones with fire arms. A glass insulator when struck flies to pieces and allows the conductor to drop on to the cross-arm, while a porcelain insulator possibly chips but stays on the pin and continues to hold the wire in place. In practice, boys-and marksmen give up trying to break the porcelain insulators after a few unsuccessful attempts. Line repairs are dangerous and difficult to make on high potential circuits, besides which they are very expensive and interruption to the service is a most serious matter.
2. Hygroscopic properties. It is a well established fact that moisture, smoke and gases prevalent in the atmosphere will condense far more rapidly and in greater volume on a glass surface than on a porcelain surface, and further, that they will adhere more tenaciously to glass than to porcelain. Glass insulators when exposed to the weather for a time, become coated with a conducting film. The surface leakage is thereby increased and continues to grow worse. On porcelain insulators, the coating so formed is much less and washes off clean during rainfalls.
There is considerable misunderstanding in reference to the meaning of the word "porcelain". A piece of true porcelain is an absolutely vitreous and homogeneous mass throughout. The body is as non-absorbent and possesses just the same insulating qualities as the glaze. A great deal of what is called porcelain is not all of this description; the body of the ware is porous and when exposed to moisture will readily absorb it and be thoroughly permeated with it. Insulators made of such material as this, when covered with a reasonably good glaze, have been used in a great deal of electrical work, but they are worse than useless for out-door work when exposed to high voltages. In dry weather they serve their purpose fairly well but in wet weather give serious trouble.
In confirmation of the above statements the following extracts from standard authorities are quoted. These were written with reference to the insulation of telegraph lines and therefore apply with much greater force to lines operated at high voltages and carrying heavy currents.
From Geo. Prescott's "Electricity and the Electric Telegraph:" "Glass is more extensively used in America than any other. The principal objections "to it are the property it possesses of becoming coated with a film of moisture in certain states of the atmosphere, and its liability to fracture. The first of these is practically more serious than the other and thus "far no effectual means of overcoming it has been discovered. The large white porcelain insulators used "on the continent of Europe appear to be superior to those of glass, but are quite expensive and this fact, "perhaps, as much as anything else, has prevented -their more general use in other countries."
From Fleming Jenkin's "Electricity and Magnetism:" "Glass of certain hinds offers the greatest resistance to conduction through its substance of any known material, but: it does not answer well for telegraphic insulation, because the surface conduction plays by far the greatest part in the leakage from a line, and glass is highly hygroscopic, i. e., it will be found covered with a moist film in most states of the weather. Porcelain of certain qualities insulates well; it is not nearly so hygroscopic as glass, and the rain runs readily from its highly glazed surface."
From Preece & Sivewright's "Telegraphy": "Nothing has yet been found which will perfectly insulate, nor can a theoretically perfect body in this respect ever be looked for. Porous substances are inadmissible on account of their absorbing moisture too readily and being thus transformed into conductors. A glaze or surface can be imparted to them, but recourse should never be had to this; only upon bodies which are in every respect suitable should a glaze be put, and then for the purpose of forming a fine, hard surface. A smooth, hard surface is indispensable; with it there is no danger of the wire being worn through by friction, nor can dust and dirt adhere to the insulator so firmly as not to be washed off by a good shower of rain."
"Glass possesses both of the qualifications named above, viz., high resistance to the passage of electricity, and a smooth, hard surface; but along with these it has one inherent disadvantage which is fatal to its employment as an insulator. It is a very hygroscopic body, that is to say, it condenses the moisture from the air very readily and in a climate such as that of England it is for this reason altogether unsuitable. The surface of a glass insulator is almost always covered more or less with a thin conducting film of moisture. It is moreover very brittle."
"Porcelain has been and is still largely employed in the manufacture of insulators. Its insulating power is high; it possesses a good, smooth surface; and provided it has been perfectly vitrified throughout so as to be homogeneous, impervious to moisture, and free from flaws, it is eminently adapted for the formation of an insulator. Porcelain, however, varies very much in its quality, and unless the manufacture has been thoroughly carried out with the greatest care, no reliance can be placed upon it."
It is a comparatively simple matter to make small pieces, but quite a different matter to make large, thick pieces of complicated shape which shall possess the qualities of true porcelain. This problem has, however, been solved by the manufacture of porcelains which fulfill the definition in all respects and insulators are .now made which will not absorb moisture, (even when unglazed) if completely immersed in water, and show just as high an electrical resistance when unglazed as when glazed. The purpose of glaz